Abstract Young Latina girls report especially low rates of physical activity, and have also shown the marked recent increases in incidence of sedentary-related diseases such as diabetes and obesity. Increasing physical activity could reduce risk factors for these diseases, yet this population has largely been excluded from interventions. This population faces cultural, gender, and SES barriers to activity, and as such needs interventions tailored to their unique needs. Our research team has extensive experience developing and implementing state-of-the-art individually tailored computer expert system-driven physical activity interventions (based on Social Cognitive Theory and the Transtheoretical Model), and has done extensive work modifying these interventions specifically for Latina women. In R21NR009864, we culturally and linguistically adapted our existing tailored intervention for sedentary Latina women and conducted a small pilot trial of the modified program (N=93). [This was recently tested in an RCT with Latinas (R01NR011295), and six-month findings showed significantly greater increases in PA in the intervention group (Mean Increase=71.66 min/wk, sd=89.92) than in controls (M=29.58, sd=84.37). This is now being tested (R01CA15995401) in a large web-based RCT.] For the current proposal, we will further modify this intervention specifically for young Latina girls and then test its feasibility and acceptability in a single arm demonstration trial. Social support, particularly parental support, is strongly predictive of greater physical activity in adolescents, thus the incorporation of parental support is likely to enhance the effectiveness of this intervention. Because the Latina women completing our current web-based intervention (R01CA15995401) will have recently increased their own physical activity, they are uniquely poised to provide invaluable support to their daughters. Therefore, to capitalize on the role of parental support, we will recruit and target young Latinas whose mothers have completed our current RCT. In Phase 1, we will conduct formative semi-structured interviews with 15 young Latinas (age [13-18]) and their mothers to guide the modification of our intervention website and materials to make them appropriate and user friendly for young Latinas, to determine the key barriers to activity unique to this population, [and to determine social support needs in mothers]. In Phase 2, we will conduct a one-arm 12-week open trial of the intervention with 25 young Latinas whose mothers have completed our ongoing intervention. Data will be collected at baseline and 12- weeks using the 7-Day PAR and accelerometers, as well as comprehensive psychosocial measures developed specifically for youth. Data from this trial will be used to determine the feasibility and acceptability of the intervention, and in-depth qualitative interviews will be conducted at completion to inform modification of the intervention for a larger trial for mothers and daughters. Such interactive web-based interventions are high reach and low cost, thus they have great potential for adoption on a larger scale and, consequently, have potential to reduce widening public health disparities.